Zener diode reference circuit independent of input voltage changes

ABSTRACT

A Zener diode voltage reference circuit whose output is maintained at exactly the Zener voltage irrespective of changes in the input voltage. More particularly, since the voltage appearing across a Zener diode fluctuates slightly with changes in the Zener current, exact regulation is accomplished by maintaining the current flowing through the diode at a constant value; furthermore, the Zener current is held constant by adding to the circuit a second Zener diode which serves to extract from the first Zener diode all the current which would cause a voltage fluctuation.

United States Patent Inventor Appl. No.

Filed Patented Assignee Eric J. Hoffman Baltimore, Md.

Feb. 19, 1969 May 4, 1971 The United States of America as represented by the Secretary of the Navy ZENER DIODE REFERENCE CIRCUIT INDEPENDENT OF INPUT VOLTAGE CHANGES 6 Claims, 5 Drawing Figs.

US. Cl 323/8, 307/285, 307/297, 323/l7, 323/22, 323/81 Int. Cl G05f 1/44 Field ofSearch 323/].4, 7,

[56] References Cited UNITED STATES PATENTS 3,255,402 6/l966 Vollnhals 323/22(T) 3,428,897 2/1969 Rew et al. 323/22X(Z) Primary Examiner-William M. Shoop, Jr.

Assistant Examiner-Gerald Goldberg Attorneys-Justin P. Dunlavey and John O. Tresansky OUT PAIENTEDHAY 41971 7 8,577,062

2 W m z F I G. 2

PRIOR ART OUT FIG. 1 In PRIOR ART VZ'HIBE v l SLOPE w -fly FIG. 4

R z s SLOPE I SLOPE= R R9 2 as I VI m F 16.5

INVENTOR ERIC J. HOFFMAN BY {eg AT'T EY ZENERDIODEREFERENCE CIRCUIT INDEPENDENT OF INPUT VOLTAGE CHANGES BACKGROUND OF THEINVENTION 1. Field of the Invention The present invention relates generally to voltagereference circuits and more particularly to a new and improved Zener diode voltage reference circuit that maintains an exactly constant voltage output regardless of input voltage changes.

2. Description of the Prior Art In the past, various voltage regulators have been utilized-to provide an approximate voltage reference source.-However, a voltage level output that would remain at exactly one voltage level, regardless of variations in the input-source or otherinfluences in the surrounding environment, has not been attainable. Various electronic means and methods have'heretofore been employed in the previouslydeveloped current and voltage regulators in an attempt to obtain an exactly constant output. Such means included diodes, transistors, feedback .loops, and Zener diodes. However, the instant invention, in utilizing two Zener diodes, produces an exact and'nonvariant output voltage.

SUMMARY or THE INVENTION The instant invention can be used in any low current application wherein a constant voltage reference is desired. A great numberof possible uses can be imagined, and mentioning only a small number of them, the instant invention would improve the operation of analog-to-digital converters, digital-to-analog converters, thermal reference regulators and voltage comparators. It is therefore obvious that the subject invention -.could be used in a-plurality of applications.

It is therefore the primary object of the present inventionto ,provide a new and-improved voltage reference circuit.

A further object of the invention is to provide a voltage reference circuit that will'yield a constant voltage output regardless of changes in the input voltage.

Another object of the invention is to provide .a voltage reference circuit utilizing two Zener diodes.

A still further object of this invention is to provide a voltage reference circuit whereby one Zener diode serves to extract fluctuations of the input current such that the other Zener diode will receive only exact current input.

An additional important object of the invention :is to provide a voltage reference circuit wherein the variationofoutput voltage with input voltage can be varied in direction and magnitude by merely adjusting the impedance of one of the circuit resistors. In particular, it is possible to obtain zero variation of output voltage with input voltage.

Still another object of this invention is to provide a voltage reference unit utilizing two Zener diodes such that the combination of the individual temperature effects of each diode is .to yield a total flattemperature characteristic.

Other objects, functions and characteristic features of the instant invention will become more apparent as the discussion of the subject invention proceeds and will become obvious in part from the detailed description of the preferred embodiments and in part from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS diode l and resistor 2 and in FIG. 2, there is shown a curve of the Zener current versus the input voltage of the circuit shown in FIG. 1. It can bereadily observed that as the input voltage increases, so does the current of the Zener diode 1, assuming,

of course, the voltage has exceeded the breakdown voltage level of the Zener diode 1. Therefore, for any variation in input voltage there is an accompanying variation of the Zener 5 diode current which, in turn, affects the stability of output voltage.

Referring now to FIG. 4, there is shown the voltage-current curve of a voltage reference circuit similar to that shown in FIG. 1. It should be noted, however, that the voltage required totum on the Zener diode 1 shown in-FIG. l, is V,; and the voltage required to turn on the Zener diode whose characteristics areshown in FIG. 4 is V +V (V being greater than V,).

Itshouldnow be evident that if a voltage reference circuit whose characteristics are shown in FIG. 2 is connected to a voltage reference circuit whose characteristics are shown in FIG. 4 (the'FIG. 4 reference being inverted), the main Zener diode in the resultant circuit has the characteristics shown in FIG. 5. In said resultant circuit no current would flow through the main diode until the. input voltage reaches V (that voltage necessary to run on the main diode); whereupon the current through this diode increases, unaffected by the second diode, until the input voltage reaches V +V (that voltage required to turn on the second diode). Once both diodes are conducting the current flowing through the main diode would remain at a constant value, since the second diode isconnected in the circuit in such amanner that all unwanted current is extracted from .the-maindiode. And, as .noted above, since the current flowing through the main diode is held at a constant value, the voltage across this diode is constant.

In reference now to FIG. 3, there is shown the voltage reference circuit of the present invention, said reference circuit having the characteristics as shown in FIG. 5. In FIG. 3 there are serially connected circuit input terminal 3, secondary extractor Zener diode 4, transistor 5, and resistor 6, the input of said resistor 6 being connected to the emitter of said transistorand the output of said resistor 6 being connected to ground 7. The purpose of the transistor 5 is to invert theeffects of Zenerdiode 4. Also included in the improved voltage reference circuit is serially connected variable resistor '8, resistor-9, main Zener diode l0, and ground connection 11. Resistor 9 is included in the preferred embodiment to prevent excessive input current from being drawn into input terminal 3 when the voltage at said input'terminalexceeds the operating range of transistor 5. An output terminal 12 is electrically connectedbetweenresistor 9 and .mainZener diode 10. Also, the collector of the transistor 5 is connected in electrical parallelism between resistor 8 and resistor 9,.as shown at 13. Finally, the resistor 8 is connected to the input terminal 3.

As noted above, the main Zenerdiode does not conduct until the voltage reaches the breakdown voltage level of said diode. When said breakdown voltage level is reached, the Zener current .increases.proportionately.with a slope equal to s-la and continues thereln until voltage is obtained, whereupon the Zener current increases with a slope of through the main diode 10 is drawn to ground 7 through the collector-.emitter.path of the connecting transistor .5.

if the output voltage is desired to exhibit small changes for accompanying changes in voltage input, this can be accomplished by varying the resistance of resistor 8. in this manner, the slope of the curve in FIG. that is normally equal to zero can be changed to reflect small changes in voltage regulation.

Another added effect that the combination of the two Zener diodes exhibits is a fiat temperature characteristic, such that the temperature variation in one Zener diode is compensated for by the other diode and the base-emitter voltage of transistor 5 whereby for nominal changes in operating temperatures, there are little. if any, accompanying changes in the output voltage.

For determining the value of current through the main Zener diode 10, a nodal equation at 13 would be;

Where the V is the voltage at input terminal 3,

V I is the voltage at 13 V is the voltage across Zener diode V is the voltage across Zener diode 4 R is the impedance of resistor 8 R is the impedance of resistor 6 R is the impedance of resistor 9 Assuming 0: equals 1, and rearranging, the following equation is obtained:

where 1 is the main Zener diode current.

However, for

Ram

and by making ti ti the main zener diode current becomes V2 VBE V1 R 9 R In conclusion it will be clear that the main Zener diode current will be independent of the input voltage and accompanying fluctuations.

in a practical embodiment, where a is slightly less than unity, and the Zener diodes have small internal resistances, R is set slightly larger than R to obtain the desired result, namely sistance means and ground.

an output terminal connected to the junction of said resistance means and said first Zener diode. and

second circuit means connected in parallel with said first circuit means and being responsive to variations in said variable input voltage for diverting said voltage variations away from said first circuit means to ground to insure that said first circuit means receives a constant current effective to insure that the voltage appearing at said output terminal remains constant regardless of variations in said input voltage.

2. The regulator circuit as defined in claim 1 wherein said resistance means consists of:

a variable resistor connected to said input current supply,

and

a first. fixed resistor serially connected between said variable resistor and said first Zener diode, whereby adjustment of said variable resistor causes said regulator circuit to provide a variable amount of voltage regulation in both magnitude and direction.

3. The regulator circuit as recited in claim 2 wherein said circuit means comprises:

a second Zener diode having an anode and cathode and having a greater voltage breakdown level than that of said first Zener diode,

a transistor having a base, collector and emitter,

a second fixed resistor. A

the cathode of said second Zener diode being connected to said input supply and the anode of said second Zener diode being connected to the base of said transistor, and

the collector of said transistor being connectedat the junction of said variable resistor and first fixed resistor, and the emitter of said transistor being connected by said second fixed resistor to ground.

4. The circuit as recited in claim 1 wherein said resistance means consists of first and second serially connected resistors, and wherein:

said second circuit means comprises;

a second Zener diode having an anode and cathode and having a greater voltage breakdown level than that of said first Zener diode,

a transistor having a base, collector and emitter, and

a third fixed resistor,

the cathode of said second Zener diode being connected to said input supply and the anode of said second Zener diode being connected to the base of said transistor,

the collector of said transistor being connected at the junction of said pair of serially connected resistors and the emitter of said transistor being connected by said third fixed resistor to ground.

5. The circuit as defined in claim 4 wherein the values of that resistor of said serially connected pair which is connected to the input supply is approximately equal to the value of said third fixed resistor such that a constant level of output voltage 

1. A voltage regulator circuit for producing constant output voltage from a variable input voltage comprising: first circuit means connected between said variable input voltage and ground, said first circuit means including: a resistance means connected to said variable input voltage and a first Zener diode serially connected between said resistance means and ground, an output terminal connected to the junction of said resistance means and said first Zener diode, and second circuit means connected in parallel with said first circuit means and being responsive to variations in said variable input voltage for diverting said voltage variations away from said first circuit means to ground to insure that said first circuit means receives a constant current effective to insure that the voltage appearing at said output terminal remains constant regardless of variations in said input voltage.
 2. The regulator circuit as defined in claim 1 wherein said resistance means consists of: a variable resistor connected to said input current supply, and a first, fixed resistor serially connected between said variable resistor and said first Zener diode, whereby adjustment of said variable resistor causes said regulator circuit to provide a variable amount of voltage regulation in both magnitude and direction.
 3. The regulator circuit as recited in claim 2 wherein said circuit means comprises: a second Zener diode having an anode and cathode and having a greater voltage breakdown level than that of said first Zener diode, a transistor having a base, collector and emitter, a second fixed resistor, the cathode of said second Zener diode being connected to said input supply and the anode of said second Zener diode being connected to the base of said transistor, and the collector of said transistor being connected at the junction of said variable resistor and first fixed resistor, and the emitter of said transistor being connected by said second fixed resistor to ground.
 4. The circuit as recited in claim 1 wherein said resistance means consists of first and second serially connected resistors, and wherein: said second circuit means comprises; a second Zener diode having an anode and cathode and having a greater voltage breakdown level than that of said first Zener diode, a transistor having a base, collector and emitter, and a third fixed resistor, the cathode of said second Zener diode being connected to said input supply and the anode of said second Zener diode being connected to the base of said transistor, the collector of said transistor being connected at the junction of said pair of serially connected resistors and the emitter of said transistor being connected by said third fixed resistor to ground.
 5. The circuit as defined in claim 4 wherein the values of that resistor of said serially connected pair which is connected to the input supply is approximately equal to the value of said third fixed resistor such that a constant level of output voltage will be obtained.
 6. The circuit as described in claim 4 wherein the value of that resistor of said serially connected pair which is connected to said input supply is variable over a range which includes the value of said third fixed resistor. 